Foams comprising polyolefins are used extensively in the packaging and construction industries, and are typically produced by extrusion processes that are well known in the art.
In such processes, a polyolefin resin is introduced to an extruder, typically in the form of pellets. Common polyolefins employed for making the foams include polyethylene, such as low density polyethylene (LDPE), and polypropylene (PP). Once in the extruder, the polyolefin melts and a blowing agent is admixed with the molten polyolefin (typically under high pressure). The blowing agent may be introduced to the extruder with or separate from the polyolefin. The extruder then pumps the melt mixture (i.e. the molten polyolefin and blowing agent) through a die and into a region of reduced temperature and pressure (relative to the temperature and pressure within the extruder).
Generally, the region of reduced temperature and pressure is at ambient conditions. Upon the melt mixture being exposed to reduced pressure, the blowing agent promotes formation of a plurality of gas bubbles, which upon solidification of the molten polymer give rise to a plurality of cells within the polymer to thereby form the foam.
Depending upon the conditions/reagents employed, the resulting foam may have an open cell or closed cell structure. Open cell structured foams have inter-connected pores and generally exhibit a relatively low compressive strength. Closed cell structured foams have pores that are isolated within the polymeric matrix and are therefore not inter-connected. Such foams typically exhibit a relatively high compressive strength.
Polyolefins are particularly well suited for producing in an effective and efficient manner foamed products that exhibit excellent properties. However, with an ever increasing emphasis on sustainability and the environment, there is a concerted effort mounting to develop foam polymer products that are less reliant upon using petroleum derived polymers such as polyolefins.
Foamed products made from renewable materials such as starch have been developed. However, such products generally exhibit inferior properties compared with their polyolefin counterparts.
Polyolefin/starch blends have also been developed. However, combining relatively hydrophilic starch with relatively hydrophobic polyolefins to produce a polymer blend with good mechanical properties has proven difficult in practice. In particular, melt processing starch with a polyolefin generally results in the formation of a polymer blend having a multi-phase discontinuous morphology. Such morphologies are typically unstable and exhibit high interfacial tension, the likes of which are not particularly suitable for use in the formation of foam products.
An opportunity therefore remains to address or ameliorate one or more disadvantages or shortcomings associated with conventional foams and/or their production, or at least to provide a useful alternative foam product and/or method of production.